Digitizers, as known in the art, are very useful for translating the position of a point on a plan or drawing into coordinates recognizable by a computer. In the latter form, the computer can make use of the position information in any number of useful ways. Unlike the position information from a mouse, which is always relative to its immediately previous position on a support surface, the digitizer position information is relative to a grid of conductors incorporated into its work surface. It becomes, therefore, absolute with respect to the work surface and any plan or drawing mounted thereon in a designated orientation. As a result, the position information is accurate enough not only to edit graphic displays, but also to control manufacturing processes according to scale drawings placed on the work surface, and to control navigation of air and water craft according to charts.
A number of well known digitizer systems make use of a movable coil and a work surface defined by a receiving grid of conductors. The coil may be disposed within the tip of a pen-shaped instrument so that the pen point accurately locates the effective coil center. Alternatively, the coil may surround a transparent disc with a set of cross hairs etched thereon to mark the coil center in what is known as a cursor. The receiving grid normally comprises a set of parallel conductors uniformly spaced along the work surface in what may be called the x direction and another set similarly disposed along the work surface in the orthogonal y direction. An oscillator applies an ac signal of predetermined frequency and amplitude to the coil, which is inductively coupled to the conductors of the grid.
In accordance with well known principles of electromagnetic theory, ac electrical signals are induced in the grid conductors at a magnitude and phase that depend on the location of the coil relative to the conductors. Generally, the signals induced in the conductors will have a magnitude that varies from zero at the coil center to a maximum at the coil periphery and tapering off beyond. Further, the phase of the signals induced in conductors at one side of the coil will be the opposite of (180 degrees displaced from) that of signals induced in conductors at the other side.
The conductors in each set are accessed in a predetermined order through the use of multiplexer circuitry to couple the induced signals to detection circuitry. By identifying the two adjacent conductors in each set in which the induced signals are of opposite phase, the detection circuitry coarsely locates the pointer between them. The exact position of the pointer between the indicated conductors may then be determined by the relative magnitudes of induced signal in the adjacent conductors.
One disadvantage of these known systems arises from the necessity of supplying sufficient power to the coil in the pointer. Normally, a cable connects the ac signal to the coil, tethering the pointer to the base electronics associated with the work surface. Additional wires in the cable connected to switches on the pointer allow the operator to supply additional information to the base electronics. For example, the information that the coil is at a point where coordinates are desired can be so signalled. Such cables, however, restrict free movement of the pointer across the digitizer work surface. Alternatively, the oscillator may be located in the pointer together with a battery for power. In this case, the switches on the pointer can cause phase shifts in the oscillator signal that will be recognized by the base electronics. Such batteries, however, tend to be heavy and bulky, or else expensive and short-lived.
A digitizing system that requires neither cable nor battery is disclosed in U.S. Pat. No. 4,878,553 which issued on Nov. 7, 1989 to T. Yamanami et al. According to the disclosure of that patent, an ac signal is first applied to a selected receiving grid conductor. A tuned circuit comprising a coil and a capacitor located in the pointer is inductively excited by the ac signal on the selected conductor to resonate at the same frequency. The ac signal is then removed from the selected conductor, and, after a sufficient time interval for confusing transients and clutter to subside, the conductor is connected to detection circuitry. The tuned circuit in the pointer continues to ring, and it serves as the ac source for detection as in the prior art. While this disclosed system dispenses with the unwanted cable and battery, it has other disadvantages. Since each selected conductor must first be excited directly by the oscillator and then inductively by the ringing tuned circuit, the time taken to process each conductor is relatively long. This significantly slows down the rate at which positions can be determined. Furthermore, as soon as the ac signal is disconnected from the selected conductor, and therefore during the whole time the detector is making its measurements, the amplitude of the coil signal decays at an exponential rate. This can introduce inaccuracies into the position determination process.
An object of our invention is to provide a digitizer system having an untethered passive pointer that can make use of a full-time driving signal.